To restore the colors in all their variety, one proceeds in general by additive synthesis of at least three complementary colors, especially red, green and blue. In a chromaticity diagram, the subset of available colors obtained by mixing different proportions of these three colors is formed by the triangle formed by the three coordinates associated with the three colors red green and blue. This subset constitutes what is called a gamut.
The majority of color display devices operates on this three-color principle: each pixel consists of three sub-pixels, one red, one green and one blue, whose mixture with different intensities can reproduce a colorful impression.
A luminescent or backlit display such as a computer LCD screen has to present the widest possible gamut for an accurate color reproduction. For this, the composing sub-pixels must be of the most saturated colors possible in order to describe the widest possible gamut. A light source has a saturated color if it is close to a monochromatic color. From a spectral point of view, this means that the light emitted by the source is comprised of a single narrow band of wavelengths. We recall that a highly saturated shade has a vivid, intense color while a less saturated shade appears more bland and gray.
It is therefore important to have light sources whose emission spectra are narrow and therefore of saturated colors.
For example, in the case of a color display, the red, green and blue sub-pixels composing it must have a spectrum maximizing the gamut of the display system, which amounts to exhibiting the narrowest possible emission from a spectral point of view.
It is possible to distinguish two types of polychromic light-emitting displays:                Backlit displays, in which a white light coming from the backlighting is filtered by color filters and whose intensity is controlled by a liquid crystal system: these are the liquid-crystal displays (LCD),        Directly emissive displays, in which each pixel consists of at least three sub-pixels corresponding to three basic colors. Each sub-pixel is a light emitter independently addressed, often through a matrix or multiplexed system, which emitted light intensity is then directly set. This is the case of plasma screens and light-emitting diodes screens such as OLEDs screens (for “Organic Light Emitting Diode”). These devices use a material emitting light in response to an excitation.        
In LCD screens, the color of the pixels is determined by the filtering of a white primary source by red, green and blue filters. The spectra of the three sub-pixels therefore correspond to the multiplication of the emission spectrum of the primary source, which is usually an array of white LEDs or a cold cathode fluorescent tube, by the transmission spectrum of the filters used. The fact of optimizing the spectra of the primary light source or of the color filters therefore allows improving the gamut. However, most of the light emitted by the white primary source is either reabsorbed by the polarizers and color filters that make up the screen, or deflected by diffusion and waveguide effect in the different layers. Thus it does not reach the observer, which severely limits the energy efficiency of liquid-crystal displays. It therefore requires, to limit power consumption, to seek a gamut-brightness compromise.
To increase the color gamut and brightness of the screen without significant change of the filters and the primary light source, it has been recently proposed to add a fluorescent film containing colloidal quantum dots between the light source and the pixels in order to modify the spectrum of the light coming from the source after passing through the film in question and thus to enhance the saturation of the three sub-pixels4, 5. However, this solution, even though improving the gamut, decreases the brightness of the screen.
It was also proposed to replace the filters by green, red and blue wavelength converters which absorb the primary light, blue or ultraviolet for instance, and which retransmit the specific color of each converter. For this, a material containing fluorophores which absorb the light from a primary excitation source and re-emit it at a higher wavelength is used. However, this solution has issues of stability, fluorescence efficiency, and spectral finesse of the fluorophores used in said wavelength converters.
The directly emissive displays, such as displays composed of light-emitting diodes, are potentially lower in energy consumption; there is little or no loss by filtering. However, when using semi-conductor layers, such as in inorganic diodes or polymer layers as in the case of OLEDs, light losses by total internal reflections in said layers, reduce the total light that reaches the observer.
In directly emissive displays, the nature of the excitation can be various:                Electric, by charges injection as in the case of organic or inorganic light-emitting diodes.        Optical, by absorption of photons of wavelength shorter than the emission wavelength, as in the case of wavelength converters or plasma screens.        
Many emissive materials have been proposed to try to cover the entire visible spectrum.
Thus, organic fluorophores, present for example in OLEDs have a high quantum yield in the visible, commonly greater than 90%. They are generally poorly stable, degrading for example due to oxidation or radiations, which reduces the lifetime of the containing devices. Moreover the width of the fluorescence spectra can be quite large, which does not allow to obtain a large gamut. Finally, the optimal excitation wavelength can be different for each fluorophore, making their integration into a system with a common excitation source difficult.
Oxides or complexes of the rare earths are emissive materials commonly used, such as in plasma screens and OLEDs. In this case, the emissive material is much more stable as it is weakly sensitive to oxidation. The width of the emission peaks can be very small, of the order of ten nanometers, but the absorption cross section of these materials is low, which may require the use of large quantities. Moreover their emission wavelength is not tunable, because it is defined by the material, for example the rare earth complex used. This is an important limitation, which does not allow this type of transmitters to cover the entire visible spectrum.
The emissive materials of plasma screens or OLEDs sometimes include a transition metal oxide. As for the rare earth oxides, the fluorescent material is very stable as it is weakly sensitive to oxidation. However, the fluorescence spectral width is very high, typically from fifty to several hundred nanometers, which does not allow to generate saturated colors and thus to present a high gamut.
Semiconductor nanoparticles, commonly called “quantum dots”, are an alternative as emissive material. Said objects have a narrow fluorescence spectrum, approximately 30 nm full width at half maximum, and offer the possibility to emit in the entire visible spectrum as well as in the infrared with a single excitation source in the ultraviolet8, 9. However, they do not allow to optimize the light received by the observer and thus the energetic efficiency of the device. In this case an improvement of the gamut of polychromic displays requires a finesse of the emission spectra that is not accessible for quantum dots.
The object of the present invention is thus to provide a new light-emitting device allowing a great spectral emission finesse, a perfect control of the emission wavelength, the directivity and/or polarization of the emitted light. The present invention thus significantly improves the brightness and color gamut of displays composed of said devices.